Recently, ink-jet printers have become widely used by virtue of lower noise and relatively less running cost, and also color printers printable on regular paper have been actively put into the market. However, it is very difficult for ink-jet printers to satisfy all of common properties such as color reproducibility, endurance, lightfastness, image-drying property, non-feathering or non-bleeding of images, double-face printing and discharge stability, thus inks are selectively employed considering important properties for respective applications.
The inks for ink-jet printers are typically based on water and contain colorants like dyes or pigments and humectants such as glycerin to prevent clogging. The colorants are usually dyes in view of superior color-producing property and stability. However, lightfastness and water resistance of images formed from inks containing dyes are relatively inferior. Specifically, the water resistance of images is unsatisfactory on regular paper, although it may be somewhat improved on dedicated ink-jet recording paper having an ink-absorbing layer.
In order to solve these problems, recently, alternative recording inks have been investigated in which pigments, in place of the dyes, such as organic pigments and carbon black are micronized with surfactants or dispersants and dispersed into media such as water. Patent Literatures 1 and 2, for example, disclose methods for improving ejection stability by way of micronizing organic pigments into a particle diameter of 50 nm or less using specific dispersants. Patent Literature 3 discloses an ink-jet ink using a specific organic pigment and an anionic dispersant.
However, it is impossible to micronize organic pigments into below their primary sizes regardless of any dispersants. Furthermore, it is very difficult to reduce the particle size of organic pigments into 50 nm or less without degrading color tone. Furthermore, in cases where pigment dispersants of lower-molecular weights are employed, there exists a problem that dispersion stability of pigments is inferior, pigments are likely to coagulate with aqueous solvents in inks; in cases where pigment dispersants of lower-molecular weights are employed, the dispersibility is likely to be poor due to higher velocities of pigment dispersions. There exist also a problem that printed matters from these inks are insufficient for scratch resistance and water resistance.
Since ink-jet recording requires stable discharge of ink droplets from a minute nozzle of ink-jet recording heads, it is necessary that the ink is far from solidification due to drying at orifices of the ink-jet recording heads. In cases where inks of dispersed organic pigments are utilized for ink-jet recording, however, dispersion stability of pigments may be degraded under long-term storage in particular, thus the resulting coagulation of pigments may lead to clogging of ejection nozzles of ink-jet recording heads or non-ejection of inks. Especially when the printing is suspended for a long period, nozzle clogging is likely to occur, and the thickened inks may deposit within maintenance devices such as nozzle caps or suction tubes, which possibly hindering the function of maintenance devices. Furthermore, when printing is temporarily suspended or nozzles are stopped for a short period due to blanks or a single blank in printing documents, there often occur problems such as printing failures or intermittent discharge-failures due to distorted-injection direction of ink droplets.
As for the other dispersing methods of organic pigments, so-called surface-modified recording inks are proposed in which pigments are stably dispersed, with no dispersant, by way of modifying the pigment-particle surface into hydrophilic with carboxyl group, carbonyl group, sulfone group, hydroxyl group, and the like. As for black recording inks, for example, so-called surface-modified carbon blacks have been developed in which carbon blacks are stably dispersed, with no dispersant, by way of introducing hydrophilic groups on the carbon surface. In addition, as for inks of color pigments, so-called surface-modified color pigments have been developed in which the color pigments are stably dispersed with no dispersant. However, these surface-modified recording inks typically lack scratch resistance on regular paper or dedicated gloss paper, and represent poor water resistance.
In order solve the problems described above, there is proposed an ink-jet ink containing microcapsules or emulsion where pigment particles are coated with a resin. In accordance with such a proposal, since the pigment particles are coated solidly by the resin, dispersion may be stable for a long period and the ejection stability may be improved; however, it is expensive and difficult to make the dispersion particle diameter into 150 nm or less even pigments with a primary particle diameter 50 nm to 100 nm are coated with the resin.
Patent Literatures 4 and 5 describe a proposal in which a colorant may be provided with a nanometer-particle diameter and a uniform particle size distribution by way of depositing a dye onto the surface of metal oxides and then coating it with an organic compound having an ionic group, and the resulting colorant can be utilized for ink-jet inks. In accordance with this proposal, water resistance may be improved compared to inks utilizing dyes; however, lightfastness tends to be inferior to the inks utilizing dyes, thus the resulting inks are inappropriate for actual use.
As such, in cases where dispersants are utilized for dispersing pigments or pigment surfaces are treated to be hydrophilic, a resin may be added to inks in order to complement the lack of water resistance and scratch resistance; however, it is typically impossible to obtain sufficient water resistance when water-soluble polymers are utilized. For this reason, methods using water-dispersible resins are proposed; however, the water resistance is still insufficient since the printed resin particles on recorded matters may easily disperse into water again.
Recently, composite pigments are proposed in which an organic pigment is coated over inorganic pigment particles. When inorganic pigment particles of about 5 to 50 nm are utilized as a core material, it is possible to obtain a colorant pigment of less than 100 nm even when being coated with an organic pigment (see Patent Literatures 6 and 7). Complex-pigment colorants of 10 to 20 nm are commercially available already. Application of these composite pigment particles to paints or ink-jet inks are also proposed as disclosed in Patent Literatures 8 and 9. However, these composite pigment particles also suffer from the inferior dispersion stability in particular under long-term storage and insufficient water resistance and scratch resistance of printed matters because of containing no fixing resin.
Ink compositions containing resins are also proposed, for example, an ink containing a pigment and a water-dispersible resin dispersed in water as disclosed in Patent Literature 10; an ink containing a pigment dispersed in polymer emulsion dispersion of a water-insoluble resin as disclosed in Patent Literature 11; and an ink using an emulsion having a specific film-forming temperature as disclosed in Patent Literature 12.
In addition, there exist many proposals with respect to addition of polymer dispersants in order to improve fixing ability of the complex pigment particles as disclosed in Patent Literatures 9, 13, 14, 15, 16, 17, 18 and 19. In these proposals, polymer dispersants are expected to enhance the fixing ability. However, these polymer dispersants typically suffer from insufficient water resistance due to their water solubility even though the scratch resistance may be sufficiently improved.
Accordingly, such materials are demanded currently as a pigment dispersion that can exhibit superior pigment-dispersion stability under long-term storage; a recording ink, containing the pigment dispersion, that can represent superior ejection stability without head clogging at printing, that can bring about superior image durability such as water resistance and lightfastness, that can provide high-quality images with superior color tone on not only dedicated recording paper but also regular paper, and that is suited to ink-jet recording; and the related technologies.
As for inks for the ink-jet recording at homes and offices, aqueous inks are typically utilized that are based on water and contain colorants, humectants such as glycerin, wetting agents to control permeability into recording paper, surfactants and other optional additives.
The ink-jet recording of aqueous inks is fixed typically through permeation of the aqueous inks into recording media, therefore, dedicated ink-jet recording paper has been developed that is provided with enhanced absorbability, fixing ability for coloring components on paper surface, and protection ability for the coloring components. However, the dedicated ink-jet recording paper is relatively expensive since its production process involves multiple coating processes after paper making and is inferior to the regular paper in terms of recycle ability because of containing many processing chemicals; as such, sufficient image quality on regular paper is desired currently.
The regular paper represents inferior ink-absorbability and less assistance in ink performances compared to dedicated ink-jet recording paper, therefore, there exist such problems in recording on regular paper as (1) occurrence of feathering, (2) occurrence of bleeding, (3) reduction of density, (4) reduction of color developing property, (5) reduction of water resistance, (6) reduction of light resistance, (7) reduction of gas resistance, (8) reduction of fixing property, (9) show through of ink, and the like. It is important to solve these problems in order to carry out the ink-jet recording on the regular paper.
In recent years, pigments have been often used for ink-jet inks through improvement of pigment-dispersibility and/or micronization of particle diameter. The pigment-dispersibility has been improved through improving self-dispersion stability i.e. imparting hydrophilicity by way of surface modification such as oxidation, sulfonation, or graft polymerization on pigment surface in addition of conventional pigment dispersion using surfactants and/or water-soluble resins. These pigments may improve the above-described (5), (6) and (7); the concentration and coloring property of the modified pigments are inferior to those of dyes, and also the ejection stability, long-term preservability and re-dispersibility of the pigment inks are inferior to those of dye inks. Thus, in cases where pigments are used as colorants, it is important to enhance ink density, color development, and reliability. In order to address these problems, for example, a number of ink-jet recording inks are proposed that contain colored polymer particles, in particular emulsion of polyester or vinyl based polymer particles (see Patent Literature 20, non-Patent Literature 1). These proposals encompass inks containing a colorant-including resin dispersion, in which the colorant is included in a water-insoluble, aqueous dispersible resin. When color organic pigments are used as the colorant, the image density and the color reproducibility under conventional ink formulations on regular paper are superior to those of pigment inks with water-soluble dispersants.
Heretofore, control of ink permeability into the paper has been investigated in order to reduce feathering and bleeding, to enhance printing density and color development and to suppress show through. A super-permeable ink-jet ink is commercially available of which the surface tension is adjusted to below 35 mN/m to enhance permeability into paper. Such inks effectively reduce the bleeding and are easily dried on regular paper, meanwhile there exist such deficiencies as feathering, lower print density and lower print quality. On the other hand, a slow-permeation ink is also commercially available of which the surface tension is adjusted to above 35 mN/m thereby to slow permeation into paper and to hold the ink on surface area of the paper, which may effectively lower feathering, increase print density and color development, and to reduce show through.
However, the lower permeability degrades remarkably the drying ability after printing on the regular paper, thus resulting in deterioration of fixing ability and/or bleeding between colors in cases of multiple-color print. In view of these problems, a complex ink set has been developed and utilized in which a slow-permeation ink and a super-permeable ink are combined to suppress the bleeding between colors thereby to assure image quality. However, in cases of double-sided printing using slow-permeation inks, a period is required to wait ink-drying after printing, which deteriorate productivity of the double-sided printing. A printing apparatus is also commercially available which is equipped with a device to heat paper before and after the printing in order to enhance ink-drying ability (see Patent Literature 21). However, such apparatuses suffer from enlarged and complicated systems and wasteful heating energy due to additional heating devices, which diminish advantages the advantages of ink-jet recording.
Beside these proposals, ink-jet inks have been investigated variously with respect to reliability as well as image quality. Prevention of ink-viscosity increase is demanded to suppress clogging of nozzle heads. For example, Patent Literature 22 discloses that ink-voids can be prevented by way of controlling viscosity-change into below 10 times and diameter-change into below three times comparing after to before condensing concentration of inks two times thereby making pigments suppress the ink-spreading. Such inks, however, hardly produce high quality images on regular paper.
Patent Literature 23 discloses an ink in which residual after evaporating volatile contents in the ink is a liquid and the viscosity of the residual is below 10 times of the initial viscosity. However, the ink is one containing a dye thus also likely to provide poor image-quality regardless of higher reliability.
Patent Literature 24 discloses an ink in which the viscosity-increase is below 600 times comparing after to before water-evaporation at 60° C. However, the ink is also one containing a dye thus the water resistance is still insufficient in spite that reliability as well as durability are intended to increase by addition of water-soluble polymer.
Patent Literature 25 discloses that an ink with a viscosity of 5 to 15 mPa·s is appropriate in order to assure higher quality. This literature disclose that a certain compound may be favorably added to adjust initial-vaporization velocity and to arrange viscosity for reliability; which may suggest a solution to solve items (3) and (4) in terms of those containing pigments. However, this literature includes no description in terms of particle-size stability of pigments, that is, the ink may lack reliability under long-term preservation depending on ejection heads and/or nozzle sizes although it may be reliable after preservation for 24 hours.
As described above, it is necessary to use high-viscosity inks in order to assure high-quality printing under higher velocities, meanwhile it is difficult to assure reliability of high-viscosity inks.
Patent Literatures 10 and 11 disclose addition of water-insoluble resins into pigment inks for improving image quality. Patent Literature 26 discloses that images on regular paper can be improved using an ink in which the ratio of pigments to resin emulsions is 1:0.1 to 1:1 and the average particle diameter of coloring ingredients is 0.3 to 1.2 μm. Such inks containing resin emulsions may suppress feathering through lowering the bleeding, however, are likely to lack reliability as ink-jet inks due to insufficient image density. As for improvement in reliability, Patent Literature 27 discloses an ink in which a water-insoluble is added to the ink and the lowest temperature to form films is 40° C. or more; Patent Literature 28 discloses an ink in which the particle diameter of additional emulsions is no more than 50 nm. However, the reliability of these inks is likely to be still unsatisfactory and the image quality is insufficient still. Patent Literature 29 discloses improvement in printing nonuniformity by use of inks containing a pigment, saccharide or derivatives thereof, polyols containing five or more hydroxyl groups and a resin emulsion. Suck inks tend to exhibit lower permeability into regular paper and are problematic in terms of feathering and/or bleeding, fixing ability and drying period. Patent Literature 30 discloses an ink consisting of a water-dispersible resin and a self-dispersible pigment in which the solid content is 1.0 to 16% by mass and image quality is improved on regular paper. This proposal may lead to higher water resistance compared to those containing dyes; however, image supporting property may be insufficient with respect to marker pens often utilized for regular paper.
Furthermore, many investigations have been made regarding to viscosity increase along with solid-content increase when pigment dispersions and resin emulsions are added to inks. For example, Patent Literatures 29 and 31 disclose ink formulations in which viscosity-increase is not so significant in spite of solid-content increase by way of adding a resin capable of forming micelle aggregates. However, these proposals suffer from lower image quality due to insufficient permeability onto regular paper. The permeability onto paper can be explained through capillary absorption represented by Lucas-Washburn formula; and the higher is the viscosity, the lower surface tension and/or the less contact angle between paper and inks is required to achieve sufficient permeability. Patent Literatures 32 and 33 propose addition of polyvalent alcohol alkylethers into inks of higher solid-contents in order to improve permeability; however, these proposals suffer from lower image quality due to insufficient permeability in cases where the viscosity increases while the solid content increases.
It is publicly known that silicone surfactants and fluorine-containing surfactants can enhance permeability even in a minute amount and their employment has been investigated in the art. For example, Patent Literatures 34 and 35 propose ink-jet inks with fluorine-containing surfactants; Patent Literatures 36 and 37 propose inks with pigment dispersions and fluorine-containing surfactants. Patent Literature 38 also proposes an ink, with a viscosity of no less than 5 mPa·s, including a fluorine-containing surfactant and a polymer emulsion formed from polymer fine particles and water-insoluble or hardly soluble coloring materials. However, these proposals suffer from hue change along with increase of pigment content in inks, thus it is difficult to attain favorable hue at higher pigment contents, and also the water resistance and fixing ability are likely to be insufficient in cases of self-dispersion pigments.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 09-263720    Patent Literature 2: JP-A No. 09-263722    Patent Literature 3: JP-A No. 2002-88286    Patent Literature 4: JP-A No. 11-166127    Patent Literature 5: JP-A No. 2001-192582    Patent Literature 6: JP-A No. 2002-146231    Patent Literature 7: JP-A No. 2002-161221    Patent Literature 8: JP-A No. 2003-49096    Patent Literature 9: JP-A No. 2003-55591    Patent Literature 10: Japanese Patent Application Publication (JP-B) No. 62-1426    Patent Literature 11: JP-A No. 55-157668    Patent Literature 12: JP-A No. 01-217088    Patent Literature 13: JP-A No. 2003-49096    Patent Literature 14: JP-A No. 2003-105229    Patent Literature 15: JP-A No. 2003-171594    Patent Literature 16: JP-A No. 2003-192938    Patent Literature 17: JP-A No. 2003-327866    Patent Literature 18: JP-A No. 2003-268278    Patent Literature 19: JP-A No. 2003-327880    Patent Literature 20: JP-A No. 2000-191972    Patent Literature 21: JP-A No. 55-69464    Patent Literature 22: JP-A No. 2002-337449    Patent Literature 23: JP-A No. 2000-095983    Patent Literature 24: JP-A No. 09-111166    Patent Literature 25: JP-A No. 2001-262025    Patent Literature 26: JP-A No. 04-332774    Patent Literature 27: Japanese Patent (JP-B) No. 2867491    Patent Literature 28: JP-A No. 04-18462    Patent Literature 29: JP-B No. 3088588    Patent Literature 30: JP-A No. 2004-35718    Patent Literature 31: JP-A No. 2004-99800    Patent Literature 32: JP-A No. 2004-155867    Patent Literature 33: JP-A No. 2004-203903    Patent Literature 34: JP-B No. 2675001    Patent Literature 35: JP-B No. 2667401    Patent Literature 36: JP-A No. 04-211478    Patent Literature 37: JP-A No. 2003-277658    Patent Literature 38: JP-A No. 2003-226827    Non-Patent Literature 1: Applied Expansion of Functional Pigment Technology, by CMC Co., Ltd.